Oxidation Numbers

Core Concept

An oxidation number (or oxidation state) is a value assigned to an atom in a molecule or ion that represents the hypothetical charge it would have if all bonds were completely ionic.

Key Purpose: Oxidation numbers help track the transfer of electrons during chemical reactions.

Key Tips

Oxidation numbers track electron transfer in reactions.
Use rules systematically to assign oxidation numbers.
Oxidation is an increase in oxidation number, while reduction is a decrease.

Test Yourself

Assorted Multiple Choice

What is the oxidation number of sulfur in the sulfate ion, $\text{SO}_4^{2-}$?

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Rules for Assigning Oxidation Numbers

  
      Rule
      Description
      Examples
    
      Free Elements
      The oxidation number of an atom in its elemental form is 0.
      O2, N2, S8, Na all have Ox. number = 0.
    
      Monatomic Ions
      The oxidation number equals the ion's charge.
      Na+ = +1, Cl- = -1, Mg2+ = +2.
    
      Hydrogen
      In compounds: +1 (except in metal hydrides, e.g., NaH, where it is -1).
      H2O, HCl.
    
      Oxygen
      In most compounds: -2 (except in peroxides like H2O2, where it is -1, and in compounds with fluorine like OF2, where it is +2).
      CO2, SO3.
    
      Alkali Metals (Group 1)
      Always +1 in compounds.
      NaCl, K2O.
    
      Alkaline Earth Metals (Group 2)
      Always +2 in compounds.
      CaCl2, MgO.
    
      Halogens (Group 17)
      Usually -1, except when bonded to oxygen or a halogen with higher electronegativity.
      Cl2O has Cl = +1.
    
      Neutral Compounds
      The sum of oxidation numbers equals 0.
      In H2O: 2(+1) + (-2) = 0.
    
      Polyatomic Ions
      The sum of oxidation numbers equals the ion's charge.
      In SO42-: S = +6, 4 × (-2) = -8; +6 + (-8) = -2.

Steps to Assign Oxidation Numbers

Start with the rules for free elements, ions, hydrogen, and oxygen.
Use the compound's overall charge (neutral or ionic) to determine unknown oxidation numbers.
Verify by checking that the sum matches the total charge.

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